As a countermeasure for increasing an elasticity of a rubber, there have hitherto been proposed a method of increasing an amount of a filler, a method of increasing an amount of sulfur to increase the number of crosslinking points, and the like. However, these methods have a problem that the characteristics such as fracture resistance and the like are considerably deteriorated.
On the contrary, as a countermeasure for increasing the elasticity of the rubber while suppressing the lowering of the fracture resistance of rubber, there are proposed a method of adding a non-modified novolac-type or resole-type phenolic resin, and a method of adding a phenolic resin modified with an unsaturated oil such as tall oil or cashew oil or an aromatic hydrocarbon such as xylene or mesitylene. These methods are widely used for increasing the elasticity while suppressing the lowering of the fracture resistance (see, for example, JP-A-5-98081 and JP-A-2001-226528).
However, performances required in the rubber become very severer in recent years, and hence it is required to further increase the elasticity of the rubber while suppressing the lowering of the fracture resistance of the rubber. The aforementioned method of adding the non-modified phenolic resin or the modified phenolic resin is insufficient in the meaning that the elasticity of the rubber is further increased while suppressing the lowering of the fracture resistance.